The present invention concerns a timepiece including an equation of time mechanism with a display. More particularly, the invention concerns a running equation of time mechanism actuating a minute hand of the true time concentric to the set of hands of the movement.
As is known, there exists a time difference between the true solar time, which corresponds to the duration of time, which passes between two consecutive upper passages of the sun at the meridian of a same place, and the mean solar time or civil time which is the mean, over the year, duration of all the true solar days. This difference between the civil time and true time reaches +14 minutes 22 seconds on 11 February, and xe2x88x9216 minutes 23 seconds on November 4. These values vary only slightly from one year to the next.
In order to indicate the time difference between civil time and true time, certain timepieces include a so-called running equation of time mechanism, i.e. wherein the set of hands includes two concentric minute hands, one indicating the civil time, and the other the true time, the true time minute hand being actuated by an equation of time cam whose profile is determined by the difference between the mean solar time and the true time at a given moment.
The equation cam is driven in rotation at the rate of one revolution per year from a simple or perpetual calendar mechanism. The simple date mechanism is a mechanism capable of indicating the day of the week, the date of the month, the month of the year or even the phases of the moon, but which does not take account of the variation in the number of days in the month (months of 28, 29 or 30 days). In other words, the user of a watch having a simple date mechanism has to make a manual adjustment at the end of months which include less than 31 days. For example, on 28 February or 30 April, a manual adjustment will have to be made. As regards the perpetual calendar, like the simple date mechanism, it indicates the day, date, month and phases of the moon. However, unlike the simple date mechanism, the perpetual calendar mechanism automatically takes account of the length of the months (28, 29 and 30 days), without any manual intervention. A perpetual calendar mechanism thus automatically takes account of leap years.
An example of a perpetual calendar timepiece movement including a running equation of time mechanism is given by Swiss Patent No. 689 359 in the name of the Swiss company Patek Philippe.
The Patek running equation of time mechanism is supported by a support driven by a minute wheel of the watch movement. It is mainly formed of a friction wheel frictionally engaged on the support and a running equation pinion mounted on the support and including a pipe whose free end carries the true time minute hand. Two locking levers are pivoted on the support. They surround the friction wheel and are arranged such that when one of them is moved away from the friction wheel, so is the other one and, conversely, when a return spring, fixed onto the support, applies one of the levers against the periphery of the friction wheel, the other lever is also applied against the periphery of the friction wheel. One of the locking levers includes a control finger, which co-operates with a running equation lever one of the ends of which is applied against the periphery of the equation of time cam.
The running equation of time mechanism which has just been described hereinbefore operates as follows. The running equation lever is placed on the trajectory of the control finger with which one of the locking levers is provided. When the support, driven by the minute wheel, is rotating, the control finger is moved by the running equation lever, which causes the locking levers to move against the action of the return spring and the friction wheel to be released. At this moment, two racks which mesh with the running equation pinion cause the true time minute wheel to move backwards until a stud, with which one of the two racks is provided, comes into contact with the running equation lever. In a subsequent rotation of the support, the two locking levers again lock the friction wheel on the support. As regards the exact position for the true time minute hand, this is determined by the position of the positioning stud when it reaches the end of the running equation lever. The true time minute hand then returns forwards to its exact position for a given day.
The Patek mechanism advantageously enables a running equation of time timepiece to be made, i.e. having two concentric minute hands, one indicating the civil time and the other indicating the true time. This timepiece can be reset to the time by the user himself after stopping for an indeterminate period of time, since an equation cam driven by the perpetual calendar defines the determination of the time difference between the two-minute hands. The Patek construction includes, however, a significant number of moving parts, which poses the problem of its operating reliability. Moreover, because of the large number of parts of which it is formed, the Patek construction proves bulky, making it more suitable for pocket watches than wristwatches. Finally, it should also be noted that the use of a friction wheel as the main member of the Patek mechanism poses the problem of the rapid wear of this type of mechanism because of the gradual slackening of the resilient forces brought into play.
The object of the present invention is to resolve the aforementioned problems, in addition to others by proposing a running equation of time device, which is of simple construction and the operation of which satisfies all the necessary requirements of reliability.
The present invention therefore concerns a timepiece including a watch movement, a running equation of time device, as well as a date mechanism, this timepiece having a pair of hour and minute hands which indicate the civil time, and an additional minute hand which indicates the true time, the daily position of the true time minute hand with respect to the civil time minute hand being determined by the position of an equation of time cam driven in rotation at the rate of one revolution per year by a date mechanism, characterised in that the true time minute hand is driven by a differential gear which has for respective power take-off a gear train driving the civil time minute hand and a transmission member which co-operates with the equation of time cam to transmit the pivoting movement of said cam to the input of the differential gear
As a result of these features, the present invention provides a running equation of time device including a very small number of parts, such that it is compact and can be housed in any type of timepiece such as, in particular, a wristwatch. Moreover, the limited number of parts used guarantees that the equation of time device according to the invention operates reliably without breaking down. It should also be noted that the position of the true time minute hand is corrected once a day only, generally between 23 hours and midnight, when the date mechanism passes from one day to the next day. The person wearing the watch thus does not see the true time minute hand jumping at each hour change, as is the case with the running equation of time mechanisms of the prior art, which are driven by the watch motion-work. The feeling of confidence that the user experiences with respect to his watch is thus increased. Finally, the position of the minute hand indicating the true time is determined with precision insofar as the transmission member which forms one of the power take-offs of the differential gear, is directly meshed with the equation of time cam, which allows any play between said cam and said differential gear to be removed.